1. Field
The subject matter disclosed herein relates to obtaining information that may be used for determining a location of a receiver.
2. Information
Techniques such as trilateration are typically used to estimate a location of a device in a wireless cellular network (e.g., a cellular telephone network). Such trilateration is typically based, at least in part, upon the use of timing information transmitted between a mobile device and any one of several base stations. One approach, called Advanced Forward Link Trilateration (AFLT) in CDMA or Enhanced Observed Time Difference (EOTD) in GSM or Observed Time Difference of Arrival (OTDOA) in WCDMA, measures at the mobile device the relative times of arrival of signals transmitted from several base stations. These times are typically transmitted to a location server (e.g., a Position Determination Entity (PDE) in CDMA, a Serving Mobile Location Centre (SMLC) in GSM, etc.) over a wireless communication link. Here, such a location server typically computes estimates of a location of the mobile device using these times of reception. The transmit times at such base stations are typically synchronized to particular instances of times-of-day. Alternatively, the time relationship between free-running time references at these base stations can be known and used as a synchronization method. This way a base station clock need not be synchronized to common time reference. Known positions of the base stations and the times of reception are then typically used to estimate a location of the mobile device.
FIG. 1 shows an example of an AFLT system where times of reception (TR1, TR2, and TR3) of signals from cellular base stations 101, 103, and 105 are measured at mobile cellular telephone 111. This timing data may then be used to estimate a location of mobile cellular telephone 111. Such computation may be performed at mobile cellular telephone 111 itself, or at a location server if the timing information obtained by the mobile device is transmitted to the location server via a communication link. Typically, times of receptions are communicated to a location server 115 through one of the cellular base stations (e.g., base station 101, or 103, or 105) over a wireless communication link. Location server 115 is coupled to receive data from cellular base stations through the mobile switching center 113. Location server 115 may include a base station almanac (BSA) server, which provides the location of the base stations and/or the coverage area of base stations. Alternatively, the location server and the BSA server may be separate from one another while location server communicates with the base station to obtain the base station almanac for position determination. Mobile switching center 113 transmits information (e.g., voice communications) between base stations and a land-line Public Switched Telephone Network (PSTN) so that information may be transmitted between mobile cellular telephone 111 and other telephones (e.g., land-line phones on the PSTN or other mobile telephones). In some cases location server 115 may also communicate with mobile switching center 113 via a cellular link.
In another approach known as Uplink Time of Arrival (UTOA), times of reception of a signal from a mobile device are measured at several base stations (e.g., measurements taken at base stations 101, 103 and 105). This may be illustrated in FIG. 1 by reversing arrows associated with TR1, TR2, and TR3. This timing data may then be communicated to a location server to estimate a location of the mobile device.
Another technique in estimating a location of a mobile device involves measuring at the mobile device the times of arrival (aka “pseudoranges”) of signals transmitted from several space vehicles (SVs) in a satellite positioning system (SPS) such as, for example, the United States Global Positioning Satellite (GPS), the Russian GLONASS system and/or the proposed European Galileo System. Alternatively, such pseudoranges may be detected from signals received from pseudolites. Here, a pseudolite typically includes a ground-based transmitter, which broadcasts a signal encoded with a PN code (similar to the encoding of a signal received from an SV of an SPS) modulated on an L-band carrier signal, generally synchronized with navigation signals from at least one SPS. A transmitter may be assigned a unique PN code so as to permit identification by a mobile device. Pseudolites are typically useful in situations where SPS signals from an orbiting satellite might be unavailable, such as tunnels, mines, buildings or other enclosed areas, for example. Methods which use an SPS receiver to estimate a location of a mobile station may be completely autonomous (in which the SPS receiver, without any assistance, determines the position of the mobile station) or may utilize the wireless network to provide assistance data or to share in the position calculation.
Regardless of a particular technique for estimating a location of a mobile device at a location server, such a mobile device typically transmits to the location server raw measurements obtained from signals received at the mobile device. Accordingly, transmission of such raw measurement data to such a location server typically uses significant link capacity between the mobile devices and the location server. The other artifact of sending the measurements to the location server is the need to establish a wireless connection between the mobile device and the base station and the time that it takes to establish such connection.